1. Field of the Invention
The present invention relates to a method suitable for forming wiring patterns on a substrate during a printed-circuit board fabrication step.
2. Description of the Related Art
Wiring patterns for a printed circuit board are typically formed as follows. First, a resist layer comprising a photosensitive material is formed on a conductive layer (e.g., a thin film of Cu) on which wiring patterns are to be formed. Then, the resist layer is exposed with a mask having patterns of the same shapes as the wiring patterns. After patterns of the same shapes as the wiring patterns (hereinafter referred to as resist patterns) are formed in the resist layer by development, the conductive layer is etched with the resist patterns as a mask. In this manner, the wiring patterns are formed on the conductive layer.
When fabricating a double-sided (or multilayer) printed circuit board, a through hole is first formed to connect wirings formed on both sides of a substrate. The above-described steps are then repeated on both sides of the substrate. Such steps are disclosed in Japanese Unexamined Patent Publication No. 6(1994)-169146, for example.
In the above-described circuit board fabrication step, when spraying a developer on the resist layer, or when spraying an etchant on the substrate where resist patterns are formed and then etching the conductive layer, there are cases where the resist pattern on the through hole in the substrate tears. The reason for this is that the resist pattern on the through hole becomes weaker in mechanical strength than the resist patterns formed on other portions of the substrate. If the resist layer is made thicker to increase the mechanical strength, it will block the flow of a developer in the development step and the flow of an etchant in the etching step. For this reason, there are cases where accurate resist patterns cannot be obtained.